The invention relates to a method for detecting and suppressing extraneous radiation influences in radiometric measurements.
1. Field of the Invention
Radiometric measurement paths can be impaired considerably by changes in the intensity of the ambient radiation, as can occur for instance in testing of weld seams using x-ray sources or radioisotopes. In fill level measurements, this additional radiation incorrectly indicates an overly low value for the fill level, which can easily cause the containers to overflow. Systems with plastic rod detectors are especially vulnerable to such interference factors, because these detectors as a rule are operated without shielding and at the same time have a high response sensitivity, because of their relatively large dimensions.
2. Prior Art
In order to detect the occurrence of extraneous radiation influences and initiate appropriate safety provisions, additional detectors (extraneous radiation detectors) are sometimes disposed outside the radiation field of the radiometric measurement path, so that changes in the ambient radiation can be ascertained independently of changes in the intensity of the measurement path radiation.
Another method is to determine the occurrence of an extraneous radiation influence by way of specific changes (such as additional photo peaks) in the pulse level spectrum of the detector of the radiometric measurement path itself. This requires a detector whose spectrum has clearly pronounced photo peaks that allow conclusions to be drawn about the radiation energy, as is the case for instance in detectors with NaI(Tl) scintillators that are often used.
In plastic scintillators, no photo peaks occur, but from the shape of the spectra resulting from the Compton edges positioned in them, conclusions about gamma energies contained because of extraneous radiation in the radiation spectrum can be drawn.
For this purpose, for the sake of detecting synthetic gamma radiation by means of a liquid or plastic scintillator, it is known, for example from German Patent Disclosure DE 197 11 124 A1, to form the ratio of the pulse amplitude distribution of the natural gamma radiation and that of the expected or suspected synthetic gamma radiation, by setting two pulse amplitude thresholds pertaining to the pulse amplitudes, the thresholds being typical for the maximum or minimum energies that occur. From a measured counting rate ratio from the measured counting rates of the two channels for the two thresholds and from a comparison with a reference counting rate ratio in the absence of synthetic gamma radiation, it can be concluded that such synthetic gamma radiation is present if the counting rate ratio of the two channels, each extending to the respective set threshold, deviates from the reference counting rate ratio by a predeterminable amount. The goal is reached when the synthetic gamma radiation sought or suspected is detected; quantitative measurements are not contemplated.
From the course over time of the intensity, and in particular the speed of changes in intensity, it is also possible to conclude that undesired radiation factors are occurring. One such procedure is described in European Patent Disclosure EP 0 615 626 B1: Here the term xe2x80x9cextraneous radiationxe2x80x9d is understood to mean the influence of sporadically active radiation sources, as can happen in materials testing as a function of the external measurement conditions. In this version, an atypical course over time (for instance, a rapid rise) in the pulse counting rates of the photomultiplier downstream of the plastic scintillator leads to the conclusion that extraneous radiation is present. On the precondition that for this extraneous radiation a stable value after a transition time has elapsed is assumed, it is possible once this new, stabilized value is reached to calculate a correction value, with which the measurement can then be continued. Thus for a certain xe2x80x9csequence characteristicxe2x80x9d of the extraneous radiation, this method makes it possible to continue the measurement, but in principle it is limited to assessing a significant change in the pulse counting rate as a typical sign for the appearance of the extraneous radiation as defined above. As soon as the change in the pulse counting rate from an extraneous radiation source is on the order of magnitude of the xe2x80x9cregularxe2x80x9d pulse rate changes that occur during measurement, for instance when agitator mechanisms are used in the measurement path, this method can no longer be employed, since then it is no longer possible to set thresholds for the pulse rate change which when overshot or exceeded make it possible to draw a reliable conclusion about the incidence of extraneous radiation.
In simpler cases, extraneous radiation influence can also be detected by measurement values that are significantly above the measurement values in unimpeded operation or by an overdrive of the detector as a result of overly high radiation intensities; however, not all undesired extraneous radiations can be detected in this way.
Typical fill level measurement systems are basically constructed in accordance with FIG. 1 or FIG. 2. In their basic versions, they comprise either a point-type radiation source 2 combined with a detector 4 having a rod-shaped scintillator 5 (FIG. 1), or with a rod-shaped radiation source 9 combined with a detector 4 having a point-type scintillator 8 (FIG. 2). The two arrangements can also be combined and can be constructed with a plurality of radiation sources.
The exemplary embodiment of FIG. 1 is a fill level measurement system for a tank 1, comprising a point-type radiation source 2 and a scintillation detector, or probe, 4 with a rod-shaped plastic scintillator 5.
The scintillation detector 4 is connected to an evaluation device 7 via a cable 6. The evaluation device 7 can also be integrated with the detector.
In such arrangements, as the fill level increases, absorption and scattering of the measurement radiation through the medium filling the tank 1 increases, causing the intensity of the radiation striking the detector to decrease as the fill level rises.
As can be seen in FIGS. 3A-3E, the spectra produced for different fill levels by a 60Co radiator are such that as the fill level F1 . . . F5 increases, the Compton edge at 950 keV becomes shallower and shallower, the Compton peak is less and less pronounced, and the proportion of small pulses is higher and higher.
This can be explained by the fact that with an increasing fill level, the proportion of scattered radiation and zero effect radiation increases; the energy of the scattered radiation is intrinsically less than the energy of the primary measurement path radiation; and the zero effect spectrum (FIG. 4) rises comparatively sharply toward low energies.
In order to achieve the highest possible counting yield as well as high stability of the counting yield in the measurement channel, as much as possible of the entire usable part of the spectrum is utilized to derive the measurement values. As a consequence, extraneous radiation that may occur influences the counting rate in the measurement channel even if its energy is less than that of the measurement path radiation.
The present invention improves the basic method disclosed in DE 197 11 124 A1 in such a way that when there are changes in the intensity of the ambient radiation in operation of a radiometric measurement path, extraneous radiation can reliably be detected with plastic scintillators, and if possible the measurement outcome can be corrected.
This object is attained by a method for detecting and suppressing extraneous radiation influences in radiometric measurements in which measurements are performed over a useable pulse amplitude spectrum, comprising:
defining a measurement channel (MK) that extends at least essentially over the entire usable pulse amplitude spectrum;
defining at least one substitute channel (EK) that encompasses only a fraction of the usable pulse amplitude spectrum;
calibrating the measurement channel (MK) and substitute channel (EK) in terms of identical variables;
defining a relation between the measurement values, defined by the respective pulse rates of the measurement channel (MK) and substitute channel (EK) such that a substantial variation from a nominal value occurs in the value of the relation when extraneous radiation is detected; and
performing a radiometric measurement and determining the value of the relation for the measurement values obtained during the measurement.
One essential concept of the invention is that upon detection of extraneous radiation by comparison of the calibrated measurement values in a suitably specified measurement channel of the pulse height spectrum, the contribution of this channel to the overall outcome of measurement is eliminated, and instead the measurement outcome of the substitute channel, formed by the pulse height spectrum located outside a threshold defining the substantial variation, is weighted with a calibration factor, whose value is obtained from a calibration measurement in the absence of extraneous radiation. Although this does provide a statistically dictated loss of precision of the outcome of measurement, since the evaluation of pulses from a blanked-out pulse amplitude range is omitted, nevertheless it makes it possible to continue the measurement using the measurement value of the substitute channel, weighted by the calibration factor.